The two primary approaches for coal refining for the purpose of converting coal to liquids are called direct and indirect coal liquefaction. Direct coal liquefaction (DCL) reacts coal in a solvent with hydrogen at high temperature and pressure to produce liquids. DCL was first developed in Germany in 1913 and used commercially in Germany between 1927 and 1945. However, after World War II, crude oil was widely available at reasonable prices and the implementation of coal liquefaction was therefore not commercially attractive. As a result, only a small quantity of liquid fuels sold today is produced using a direct coal liquefaction process.
An important DCL technology is the H-Coal Process which was invented by Hydrocarbon Research, Inc. and is generally described in U.S. Pat. Nos. 3,519,553 and 3,791,959. The H-Coal Process uses a single ebullated-bed reactor with a hydroconversion catalyst to convert coal to liquid fuels. The ebullated-bed reactor is unique in its ability to process solids containing streams in the presence of high activity hydrogenation catalyst particles. Product oil (400° F.+) was used to slurry the coal for feeding to the reactor. Coal liquefaction took place at temperatures of 800-875° F., and hydrogen partial pressures of 1500-2500 psia. With Illinois No. 6 coal, liquid yields of greater than 50 w % on DAF coal were achieved during the multi-year demonstration program at the 200 ton per day H-Coal Pilot Plant in Catlettsburg, Ky. The H-Coal DCL Technologies demonstrated commercial readiness, however, no commercial projects proceeded as oil prices fell and oil supplies increased.
In the 1980's and 1990's research continued at a smaller scale to improve the DCL technologies and reduce investments and operating costs. The Catalytic Two-Stage Liquefaction Process (H-Coal TS) was invented by Hydrocarbon Research, Inc., as described in U.S. Pat. Nos. 4,842,719, 4,874,506, and 4,879,021, to substantially increase the yield of distillate liquids from coal. For Illinois No. 6 bituminous coal, liquid yields were increased from 3 barrels per ton of DAF coal for the single stage H-Coal Process to about 5 barrels per ton of DAF coal for the H-Coal TS Process. This was achieved by dissolving the coal feed at mild conditions while simultaneously hydrogenating the coal recycle solvent and coal liquids produced at temperatures from 600-800° F., hydrogen partial pressures of 1500-2500 psia in the presence of a hydrogenation catalyst.
In the H-Coal TS Process, the unreacted coal from the initial stage is then fed to a direct-coupled second stage reactor operating at higher temperatures of approximately 800-850° F. and at similar pressures (1500-2500 psia) with a hydroconversion catalyst, to achieve maximum coal conversion and high distillate liquid yields.
The direct liquefaction of coal, both in thermal and catalytic reactor systems, has been demonstrated to result in a low selectivity to VGO range product which is the desired boiling range product for a carbon black feedstock. However, it was discovered that the low DCL net selectivity of the VGO range product is a direct result of the large recycle of the VGO back to the liquefaction reactors and its subsequent conversion to lighter boiling range products.
In fact, Applicant has determined that the inherent selectivity of the coal conversion to VGO liquid is high and in the range of 40 to 50% of the total liquid produced when the recycle of VGO is omitted. A recycle of oil or feeding of externally derived oil is required to produce a coal/oil feedstock slurry which is suitable feedstock for the liquefaction reactors. Although a VGO boiling range material has traditionally been used for this recycle slurry oil and may be a preferred fraction, other boiling range materials can also be utilized. Therefore, applicants have discovered that the direct coal liquefaction process can be readily applied to the production of carbon black feedstock, wherein the recycle of heavy VGO boiling range material is replaced with a recycle of lighter material, preferable heavy atmospheric gas oil (endpoint of 600 to 700° F.) from the liquefaction product atmospheric still.
Additionally, carbon black feedstocks are required to be highly aromatic with a low API gravity. Moreover, the VGO derived from the modified coal liquefaction described above is not normally sufficiently dense and hydrogen deficient to meet carbon black feedstock specifications. However, as described herein, by adjusting of the level of catalytic activity in the liquefaction reactors, the required carbon black feedstock VGO product quality can be attained. The reduction in catalytic activity is achieved by adjustments to the level of hydrogen partial pressure and the amount of catalyst daily replacement to the ebullated-bed reactors.
This completely modified coal liquefaction process thus produces a large yield of VGO product meeting typical carbon black feedstock specifications. These specifications include a distillate boiling range of 650-1000° F., an API gravity of 0 to −5°, and an atomic hydrogen to carbon ratio of approximately 1.0. Relative to typical coal liquefaction, the process chemical hydrogen consumption is lower due to the heavy net yield slate and the reduction in catalytic activity. Additionally, Applicant's invention results in a large economic benefit in part because it results in the coproduction of valuable distillates in the naphtha and diesel boiling range.
The current invention therefore results in an innovative economic process for producing carbon black feedstock. Relative to the current state of the art in the U.S. (use of FCC slurry oil), the invention process will be significantly more profitable when oil prices are increased. This is partly a result, as mentioned above, of the coproduction of valuable naphtha and diesel fuels and the use of feedstocks (coal and natural gas for hydrogen production) which are inelastic to the price of oil. Applicant's process results in a carbon black feedstock cost which is nearly constant since coat prices are fairly constant. This can be compared to the current U.S. primary carbon black feedstock source/price which are based on the increasing and volatile light oil price.